1. Field of the Invention
This invention relates to surgical interbody fixation devices and in particular to a surgically implantable device for the stabilization of adjacent vertebrae of the human spine undergoing spinal arthrodesis and for the prevention of the dislodgement of spinal fusion implants used in the fusion process.
2. Description of the Related Art
When a segment of the human spine degenerates, or otherwise becomes diseased, it may become necessary to surgically remove the affected disc of that segment, and to replace it with bone for the purpose of obtaining a spinal fusion by which to restore more normal, pre-morbid, spatial relations, and to provide for enhanced stability across that segment. Performing such surgery of the spine from an anterior (front) approach offers the great advantage of avoiding the spinal cord, dural sac, and nerve roots. Unfortunately, in entering the disc space anteriorly a very important band-like structure called the anterior longitudinal ligament, is violated. This structure physiologically acts as a significant restraint resisting the anterior displacement of the disc itself and acting as a tension band binding the front portions of the vertebrae so as to limit spinal hyperextension.
Historically, various devices have been utilized in an attempt to compensate for the loss of this important stabilizing structure. These devices have assumed the form of blocks, bars, cables, or some combination thereof, and are bound to the vertebrae by screws, staples, bolts, or some combination thereof. The earliest teachings are of a metal plate attached to adjacent vertebrae with wood-type screws. Dryer teaches the use of a staple-screw combination. Brantigan U.S. Pat. No. 4,743,256 issued on May 10, 1988, teaches the use of a block inserted to replace the disc, affixed to a plate then screwed to the vertebrae above and below. Raezian U.S. Pat. No. 4,401,112 issued on Aug. 30, 1993, teaches the use of a turnbuckle affixed to an elongated staple such that at least one entire vertebral body is removed, the turnbuckle portion is placed within the spine, and the staple extends both above and below the turnbuckle and engages the adjacent vertebrae to the one removed.
Unfortunately, both staples and screws have quite predictably demonstrated the propensity to back out from the vertebrae. This is quite understandable as any motion, either micro or macro, tends to stress the interface of the metallic implant to the bone, and in doing so causes the bone to relieve the high stress upon it by resorbing and moving away from the metal. This entropic change is universally from the more tightened and thus well-fixated state, to the less tightened and less fixated state. For a staple, this is specifically from the more compressed and engaged state, to the less compressed and disengaged state. Similarly, screws in such a dynamic system loosen and back out.
The potential consequences of such loosening and consequent backing out of the hardware from the anterior aspect of the vertebral column may easily be catastrophic. Because of the proximity of the great vessels, aortic erosions and perforations of the vena cava and iliac vessels have usually occurred with unfortunate regularity and have usually resulted in death.
Therefore, the need exists for a device which is effective in restoring stability to a segment of the spine such as, but not limited to, the anterior aspect of the human spine and which will without danger remain permanently fixated once applied.